In Thunderbird, how to sign e-mails?

Signing Emails is synonymous to putting your own signature (or maybe a thumbprint or a seal) in a letter. This ensures your recipient that the message really came from you and that you have put your own mark on the e-mail for the recipient to verify. This also ensures that the message has not been tampered in between the sender and the recipient.

To use a digital certificate to sign e-mails in Thunderbird:

Compose a new Email Message.

Click the "Security" icon on the main panel (Figure 5).

Figure 5: Select "Digitally Sign This Message".

Check the "Digitally Sign This Message" icon.

You can also set to Digitally-Sign all of your outgoing messages by going to "Tools" -> "Account Settings" -> "Security" (see Figure 4 above).

Upon sending your Digitally-signed email, a "Master Password" prompt will appear (similar to the one in Figure 2 above). Enter the card/token PIN when asked for the "Master Password".

Note that the time it takes to digitally-sign an email may vary depending on the size (or if there are attachments) of the email.

When your recipient(s) has recieve(s) your digitally signed email, it should look like the one below (Figure 6) as viewed in Thunderbird.

Figure 6: A digitally signed email with an envelope icon.

There are times that your recipient(s) can view an invalid digital signature (Figure 7).